Based on the kinds of multi-state outages they suffer from, I'd say they often are all having all modems hitting the same auth/DHCP/DNS servers. Back when I was on BellSouth DSL, every time they had a DNS outage everybody would reboot their modem and that would bring down the DHCP server, but if you used third-party DNS you could remain online while the entire SE US lost DSL for an hour or two.